9.2 Anatomy of Skeletal Muscles

• Each skeletal muscle has three concentric layers of connective tissue
• Three layers of connective tissue in skeletal muscle:

  • outer epimysium
  • central perimysium
  • inner endomysium
• Epimysium is a layer of dense irregular connective tissue around skeletal muscle. It separates muscle from surrounding tissue and organs, and is connected to the deep fascia
• Perimysium is made of connective tissue fibers, and divides muscle into internal compartments
• Each compartment in the perimysium contains bundles of muscle fibers called fascicle
• Endomysium surrounds each skeletal muscle fiber, binds muscle fiber to its neighbor, and supports capillaries that supply individual fibers
• Myosatellite cells lie between the endomysium and muscle fibers are stem cells which repair damaged muscle tissue
• Tendons form when epimysium, perimysium, and endomysium come together then attaches muscle to bone, cartilage, skin, or muscle
• Aponeuroses are tendons that form thick, flattened sheets
• Skeletal muscles are called voluntary muscles because the contractions are controlled
• The neuromuscular junction is an area where motor neurons connect to a muscle fiber
• Components of the neuromuscular junction:

  • axon terminal of neuron
  • motor end plate on muscle fiber
  • synaptic cleft (space in between)
• The axon terminal plate attaches to motor end plate for muscle contractions
• For energy, blood vessels deliver oxygen and nutrients for ATP
• Sarcolemma is the plasma membrane in skeletal muscle cells
• With sarcolemma is cytoplasm, in muscle cell is sarcoplasm
• Differences:

  • skeletal muscle fibers are larger than normal cells
  • skeletal muscle fibers are multinucleate
• Myoblasts are groups of embryonic cells which fuse to form skeletal muscle fibers
• Stem cells here are called myosatellite cells and are used for regeneration
• Transverse tubules or T tubules are deep indentations in the sacrolemmal surface which form a network of narrow tubules which extend into the sarcoplasm
• Myofibrils are cylindrical fibers on the sarcoplasm of skeletal muscle fibers, shortening responsible for contraction
• Myofilaments are protein fibers making myofibrils (thick or thin filaments)
• Actin and myosin are contractile proteins in thick and thin filaments
• Surrounding the myofibril is the sarcoplasmic reticulum, a membrane complex and similar to smooth endoplasmic reticulum for other cells
• Tubules of sarcoplasmic reticulum enlarge, fuse, and form expanded chambers called terminal cisternae
• Triad is a combination of pairs of terminal cisternae with transverse tubules
• Myofibrils are organized in repeating units called sarcomeres
• Dark bands are A bands
• Light bands are I bands
• Dark bands, A, contain thick filaments with an M line, H band, and overlap zone
• M line is the center of an A band, stabilizes thick filaments
• H band is the lighter region on either side of M, contains only thick filaments
• Zone of Overlap is the dark area where thin filaments overlap thick filaments
• Light bands, I, contain thin filaments, extend between A bands of adjacent sarcomeres
• Z lines bisect I bands, mark boundaries between sarcomeres and connect thin filaments of adjacent sarcomeres
• Thin filament proteins include:

  • f-actin
  • nebulin
  • tropomyosin
  • troponin
• Nebulin extends along f-actin, stabilizing
• Troponin holds tropomyosin in place, moves tropomyosin to expose active sites during contraction
• Thick filaments are composed of myosin molecules
• Myosin structure contains a long tail and free hands with globular subunits